[R-390] Coming: Real In-rush Protection / Step Start

[Tisha Hayes]

One of my 'yobs was in analyzing transformer failures on really large
industrial devices and in substations. What we found was that when the
internal supporting structures in a transformer began to fail the
transformer core would try to rotate like a motor winding whenever a load
was switched across it at any point in the AC sine wave except where it was
at 0 volts.

In really large substation grade transformers much of the internal support
structure is made out of wood. If the transformer was exposed to an
un-limited short the entire core tries to rotate itself like a top and
tears out the wood supports, destroying the transformer.

Smaller transformers like what we have in much of our radio gear are held
together with paraffin or wax. With age and heat the paraffin dries out,
cracks and becomes loose. That "thunk" sound is the transformer winding and
laminated core trying to become a motor and to spin. Eventually this
weakens the leads, attachment points and laminates until something breaks
off or the laminates become loose and circulating currents go through the
roof, burning up the transformer.

Sometimes you can extend the life of a transformer by re-insulating the
core, vacuum epoxy bonding or refreshing/replacing the core supporting
materials (paraffin and waxes). If this is done (if it can be done), there
is no reason on why a transformer cannot last for another 50 years. Usually
by the time we find out that the transformer is shorted or opened up it is
too late.

If you have a sealed transformer that makes rattling noises when you shake
it, then it is just a matter of time until it fails unexpectedly unless you
do something to mechanically stabilize the core. Also, re-flooding the
transformer case with some sort of thermally conductive epoxy will improve
the heat transfer characteristics. Find out what types of epoxies are rated
for transformer duty before trying to homebrew something that may just end
up being a source of fuel for a big transformer fire.

The same is true when browsing at a hamfest through a box of old
transformers. If the windings are loose or the laminates are not securely
attached you need to something about that before you make that a permanent
addition to your equipment. Always test out transformers with a variac,
line fuse and load before putting them in service. If you have the means of
hi-pot testing the windings do that too.

Seeing a half million dollar transformer that has failed in a "bad" way and
has moved three or four feet on the concrete pad will make you a believer.
I am glad that I was not there at the time, the noise and light show would
of taken eight years off of my life expectancy.
-- 
Ms. Tisha Hayes/ AA4HA
-
"Life isn't about how to survive the storm, but how to dance in the rain"